Elimination of lead alkyls from automobile gasolines, in particular for the purposes of environmental protection, has meant that processes for the production of branched paraffins, in particular the isomerisation of normal paraffins to branched paraffins, is now gaining importance in the petroleum industry. Isomerisation of n-butane can produce isobutane which can be used in processes for alkylating light olefins with at least one isoparaffin to produce paraffinic cuts containing 5 to 12 carbon atoms per molecule. These cuts have high octane numbers. After dehydrogenation, isobutane can be used in etherification using methanol or ethanol. The ethers obtained (MTBE, ETBE), have high octane numbers and can be incorporated into gasoline fractions (gasoline pool).
The process for the isomerisation of hydrocarbons, preferably paraffins, containing principally 4 to 8, preferably principally 4 to 6 and more preferably principally 5 and/or 6 carbon atoms per molecule, is also a process which produces gasoline stock with high octane numbers which can be directly incorporated into gasoline fractions (pool gasoline).
This latter process has been the subject of numerous studies. Three different types of catalyst have traditionally been used to carry out the isomerisation reaction:
Friedel-Crafts type catalysts such as aluminium chloride, used at low temperatures (about 20.degree. C. to 130.degree. C.); PA1 catalysts based on a metal from group VIII deposited on alumina, generally halogenated alumina, preferably a chlorinated alumina, used at medium temperatures (about 150.degree. C.), for example those described in U.S. Pat. Nos. 2,906,798, 2,993,398, 3,791,960, 4,113,789, 4,149,993, 4,804,803, European patent applications EP-A-0 514 527, EP-A-0 661 095, EP-A-0 661 370 and EP-A-0.750.941; PA1 zeolitic catalysts comprising at least one group VIII metal deposited on a zeolite, used at high temperatures (250.degree. C. and more), for example those described in U.S. Pat. Nos. 4,727,217, 4,789,655, 4,935,578, 4,943,546 and 4,977,121 where the zeolite is a mordenite, and those described in U.S. Pat. Nos. 4,724,007, 4,780,736, 4,891,200, 5,157,198, 5,165,906, 5,277,791 and European patent application EP-A-0 601 924 where the zeolite is omega zeolite. These catalysts lead to slightly smaller octane number gains but which have the advantage of being easier to use and more resistant to poisons; nevertheless, their lower acidity means that they cannot be used for the isomerisation of n-butane.
Current processes for the isomerisation of hydrocarbons, preferably C.sub.4 -C.sub.8 paraffins, use catalysts based on platinum deposited on high activity chlorinated alumina which operate without recycling or with partial recycling, after fractionation of the unconverted n-paraffins, or with total recycling after passage over molecular sieve beds.
Operation without recycling, while simple, lacks efficiency in increasing the octane number. In order to obtain high octane numbers, the low octane number constituents must be recycled after passage either through separating columns (for example a deisohexaniser) or over molecular sieves, in the liquid or vapour phase.
The use of molecular sieves has its disadvantages, among them difficulties with using isomerisation catalysts based on halogenated alumina when it is chlorinated, because of the risks of contamination of the integrated molecular sieves with hydrochloric acid, which difficulties are sometimes overcome by using a chlorine trapping apparatus between the isomerisation zone and the adsorption zone when possible. Techniques using molecular sieves have been developed which operate in the presence of the isomerisation reactor containing catalysts based on chlorinated alumina impregnated with platinum. As an example, a non integrated system can be envisaged which employs a step for stabilising the isomerisation effluent before sending it to the molecular sieve adsorption step however, such techniques are complex and one of its disadvantages is that it is a batch process. For this reason, catalytic systems which are of lower performance have been used, based on zeolite and not using chlorine. This results in a product with an octane number which is lower by 1 to 2 points to that which would have been obtained with a catalyst based on chlorinated alumina. The laws of thermodynamics dictate that the lower the temperature, the higher the conversion of n-paraffins to isoparaffins and further, the better the conversion of C.sub.6 isomers with low octane number (methylpentanes) to C.sub.6 isomers with a higher octane number (dimethylbutanes).
In addition, "conventional" layouts must be considered which use separation columns (deisopentaniser and deisohexaniser), since separation columns can be protected from chlorine contamination. However, these layouts require a lot of equipment and consume large quantities of energy, and are thus expensive to use. A layout using a single separation column (the deisohexaniser alone) would be less expensive but could not convert all of the normal-pentane to isopentane and thus could not obtain the octane number increase obtained with recycling schemes.
Finally, U.S. Pat. No. 5,177,283, which describes the production of alkylbenzenes, mentions the possibility of associating a fractionation column which carries out distillation with a single external reaction zone for the isomerisation of C.sub.4 -C.sub.8 paraffins, the effluent from the reaction zone being returned to the column just below the point where the feed is removed from the reaction zone.